1. Field of the Invention
The present invention relates to a vacuum envelope that houses electron sources and electrodes each for gathering electrons emitted from an electron source. Particularly, the present invention relates to a flat vacuum envelope that houses field emission elements (field emission cathodes) each acting as an electron source and to a method for evacuating the same.
2. Description of the Related Art
Recently, the field emission electronic equipment, which includes a large number of micro field emission elements contained in a glass vacuum envelope and integrated in a vacuum micro-structure, is proceeding toward practical use as a vacuum microelectronic element.
As applications of the vacuum microelectronics technology, field emission devices including flat field emission display panels, pick-up tubes, electron beam lithography apparatuses, and the equivalents have been studied.
In a flat display panel embodying field emission elements, one pixel corresponds to a specific number of micro-cold cathodes (emitters).
Various types of cathodes including field emission elements, MIN-type electron emission elements, surface conduction-type emission elements, PN-junction-type electron emission elements, and others, each having a pointed end, have been proposed as the micro cold cathode.
As one most typical example, a field emission device (FED) is disclosed in xe2x80x9cNIKKEI ELECTRONICSxe2x80x9d, No. 654, Jan. 29, 1996, pp. 89-98. In the FED device, the so-called Spindt type field emission element (FED) is well known.
In the Spindt field emission element, a large number of emitter electrodes E are formed on the cathode substrate K, as shown in FIG. 6. An insulating layer SiO2 is laid over the cathode substrate K. A gate electrode GT is vapor-deposited over the insulating layer. Holes are formed in the gate electrode so as to expose the point of an emitter electrode E via each hole.
When a voltage Vgk is applied between the cathode electrode K and the gate electrode GT, the point of emitter electrode E emits electrons. An anode electrode A is placed so as to confront the cathode electrode K in the vacuum space. When an anode voltage Va is applied between the cathode electrode K and the anode electrode A, the anode electrode (A) gathers the emitted electrons. The field emission elements are arranged in group. When stripe gate electrodes are sequentially scanned while image signals are supplied to stripe cathode electrodes, the fluorescent materials coated on the cathode electrodes glow so that the display device operates as an indicator.
FIG. 7(a) is a perspective view illustrating the envelope of the above mentioned display panel. FIG. 7(b) is a side cross sectional view illustrating the envelope of the above mentioned display panel.
Referring to FIGS. 7(a) and 7(b), reference numeral 1 represents a glass substrate on the side of the anode (hereinafter referred to as an anode substrate) and 2 represents a glass substrate on the side of the cathode (hereinafter referred to as a cathode substrate). Micro-field emission elements are formed on the anode substrate so as to confront the cathode substrate. Anode electrodes are formed on the anode substrate so as to confront the cathode substrate.
The getter substrate 3 has the lower surface on which an exhaust hole 3a is formed to evacuate the inside of the envelope to a vacuum state. The getter 4 is for example, an evaporation-type getter. The getter is flashed at a high temperature after evacuating the envelope so that the inside of the envelope can be maintained to a high vacuum degree.
The juxtaposed structure of the cathode substrate 2 and the anode substrate 1 are sealed with a fritted glass 5 while the cathode substrate 2 is spaced from the anode substrate 1 by a small distance of 200 xcexcm to 500 xcexcm apart. The substrates 1 and 2 are generally arranged to be mutually shifted. Thus, the cathode electrode leads and gate electrode leads of the field emission elements can be placed to the portions where the substrates 1 and 2 do not confront from each other.
In the case of color displaying, anode electrode leads can be arranged on the cut portion (not shown) protruding toward the anode substrate.
As described above, the gap between the fringe of the cathode substrate 2 and the fringe of the anode substrate 1 are sealed with a fritted glass 5, except the getter substrate 3. An exhaust tube (not shown) is connected to the getter substrate 3 to evacuate the inside of the envelope by a vacuum pump.
In the vacuum envelope contains field emission elements, the cathode substrate 2 is separated from the anode substrate 1 by a small distance. In order to maintain the space of the envelope in a high vacuum degree, the evaporation-type getter 4 is generally disposed in the getter room. The getter 4 is vaporized by externally heating it at a high temperature. A getter mirror, which can adsorb the residual gas ousted from the electronic material or adsorbed after the evacuation step, is formed over the entire surface of the getter room.
In the flat display panel, since the very narrow space (t) of the vacuum envelope has a poor conductance to the gas flowing it, it is difficult that a vacuum pump draws the vacuum space to a high vacuum degree.
The ratio of the material for forming the existing field emission elements to the volume of the vacuum space is high. Hence, the evacuating process must be performed for a long time to bring the envelope to a predetermined vacuum degree by exhausting the remaining gas (particularly, moisture) adsorbed inside of the constituent materials.
In order to achieve a higher vacuum degree, the well-known getter flashing is performed after the evacuation process. Thereafter, the whole vacuum envelope is placed in an oven at about 200xc2x0 C. for several hours to adsorb the remaining gas in the vacuum envelope. This makes the fabricating process more complex. The long evacuating step (e.g. 220 minutes) prolongs the product completion time.
The present invention is made to solve the above-mentioned problems.
Moreover, the objective of the invention is to provide a vacuum envelope that can improve the vacuum degree in a field emission device.
Another objective of the present invention is to provide a vacuum envelope evacuating method that can effectively evacuate gas remaining in the vacuum envelope.
The objective of the present invention is achieved by a vacuum envelope comprising a first substrate formed of a glass substrate; a second substrate arranged so as to confront the first substrate; and a side wall for separating the first substrate from the second substrate by a predetermined distance to form a space therebetween; wherein a first opening used to evacuate the inside of the envelope is formed in a part of a vacuum envelope assembled by the first substrate, the second substrate and the side wall; and wherein a second opening is formed in a part of the vacuum envelope, the second opening being sealed at a different position of the vacuum envelope, different from the position of the first opening. Thus, before sealing in a vacuum state, the envelope is backed while high temperature gas is being flowed using the first opening and the second opening.
According to the present invention, the vacuum envelope further comprises field emission elements formed on the first substrate and an anode electrode formed on the second substrate so as to confront the field emission elements.
According to the present invention, the vacuum envelope further comprises a getter room placed so as to cover the first opening.
Furthermore, a method for evacuating a vacuum envelope, comprises the steps of juxtaposing a first substrate and a second substrate so as to be spaced from each other a predetermined distance apart, the first substrate on which field emission elements are formed; temporarily framing the periphery of the first substrate and the periphery of the second substrate with fritted glass to form an envelope; introducing a gas at a high temperature for a predetermined period of time to flow through the envelope; sealing an outlet, except a main opening into which the gas is introduced; and evacuating the inside of the envelope to a vacuum state through the main opening, so that the envelope is maintained in a vacuum state.
According to the present invention, the method further comprises the step of previously forming at least two openings on a side portion of the envelope temporarily assembled.
In the method according to the present invention, the gas at a high temperature is selected from the group consisting of CO (carbon monoxide), N2, H2, and a mixed gas of an inert gas and CO, N2, or H2.